13 “There has not failed from the streets thereof usury and deceit”. Usury and deceit are not hidden at least, because they are evil things, but in public they rage. For he that in his house does any evil thing, however for his evil thing does blush: “In the streets thereof usury and deceit.” Money-lending even has a profession, Money-lending also is called a science; a corporation is spoken of, a corporation as if necessary to the state, and of its profession it pays revenue; so entirely indeed in the streets is that which should have been hidden.
There is also another usury worse, when you forgive not that which to you is owed; and the eye is disturbed in that verse of the prayer, “Forgive us our debts— as we too forgive our debtors.” For what there will you do, when you are going to pray, and coming to that same verse? An insulting word you have heard: you would exact the punishment of condemnation. Do but consent to exact just so much as you have given, thou usurer of injuries! With the fist you have been smitten, slaying you seek, Evil usury!
How will you go to prayer? If you shall have left praying, which way will you come round unto the Lord? Behold you will say: “Our Father which art in heaven, hallowed be Your Name, Your kingdom come, Your will be done, as in heaven so on earth.” You will say, “Our daily bread give us today.” You will come to, “Forgive us our debts, as we also forgive our debtors.” Even in that evil city let there abound these usuries; let them not enter the walls where the breast is smitten! What will you do?
Because there thou and that verse are in the midst? Petitions for you has a heavenly Lawyer composed. He that knew what used there to be done, said to you, “Otherwise you shall not obtain.” “Verily, verily, I say unto you, that if you shall have forgiven men sins, they shall be forgiven you; but if you shall not have forgiven sins unto men, neither will your Father forgive you.” Who says this? He that knows what there is being done, in the place whereat you are standing to make request.
See how Himself has willed to be your Advocate; Himself your Counsellor, Himself the Assessor of the Father, Himself your Judge has said, “Otherwise you shall not receive.” What will you do? You will not receive, unless you shall speak; will not receive if falsely you shall speak. Therefore either you must do and speak, or else what you ask you will not earn; because they that this do not do, are in the midst of those evil usuries. Be they engaged therein, that yet do idols either adore or desire: do not thou, O people of God, do not thou, O people of Christ, do not thou the Body of Him the Head!
Give heed to the bond of your peace, give heed to the promise of your life. For what does it profit you, that you exact for injuries which you have endured? Does vengeance refresh you? Therefore, over the evil of another shall you rejoice? You have suffered evil; pardon thou; be not ye two....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)